Why do airplanes leave white trails in the sky?
The white trails are clouds made by the plane, called contrails. Jet engines release hot exhaust full of water vapor. High up, where the air is around minus 40 °C, the vapor cools in seconds and freezes into tiny ice crystals, which line up in a white streak behind the plane.
Engines breathe out water vapor
Burning jet fuel produces lots of hot water vapor, along with tiny soot particles. Right behind the engine the exhaust is far too hot for anything to form, so there’s a short gap before the trail starts.
Share this stepThe vapor cools in the icy air
At cruising altitude, around 10 kilometers up, the air is often colder than minus 40 °C. The exhaust mixes with it and cools within a second, and the vapor condenses on the soot particles into tiny droplets.
Share this stepDroplets freeze into ice crystals
Almost at once, the droplets freeze into ice crystals, the same stuff high, thin clouds are made of. Together they reflect sunlight and form the white line you see from the ground.
Share this stepPlanes draw clouds made of ice.
Hot exhausticy aircontrail
Whether a trail fades quickly or spreads into a wide cloud depends on the air. In dry air the ice evaporates in seconds. In humid air the trail can last for hours, spread out and trap heat, which adds to aviation’s climate impact.
Common follow-up questions
Why do some planes leave trails and others don’t?
It depends on the temperature and humidity at the height they fly. Lower or in drier air, the ice evaporates so fast that no trail forms.
Are contrails chemicals being sprayed?
No. The ‘chemtrails’ idea has been tested by scientists and isn’t supported by evidence. Contrails are frozen water vapor from the exhaust, the same way your breath fogs in the cold.
Do contrails affect the climate?
Yes. Long-lasting contrails that spread into clouds trap heat at night. Researchers think they warm the climate about as much as aviation’s CO2, and airlines are testing route changes to avoid them.
Can contrails predict the weather?
Somewhat. Trails that last and spread show humid air high up, which can come before a weather front.
